Inmathematics and incomputer programming, avariadic function is afunction of indefinitearity, i.e., one which accepts a variable number ofarguments. Support for variadic functions differs widely amongprogramming languages.
The termvariadic is aneologism, dating back to 1936/1937.[1] The term was not widely used until the 1970s.
There are many mathematical and logical operations that come across naturally as variadic functions. For instance, the summing of numbers or theconcatenation of strings or other sequences are operations that can be thought of as applicable to any number of operands (even though formally in these cases theassociative property is applied).
Another operation that has been implemented as a variadic function in many languages is output formatting. TheC functionprintf and theCommon Lisp functionformat are two such examples. Both take one argument that specifies the formatting of the output, andany number of arguments that provide the values to be formatted.
Variadic functions can exposetype-safety problems in some languages. For instance, C'sprintf, if used incautiously, can give rise to a class of security holes known asformat string attacks. The attack is possible because the language support for variadic functions is not type-safe: it permits the function to attempt to pop more arguments off thestack than were placed there, corrupting the stack and leading to unexpected behavior. As a consequence of this, theCERT Coordination Center considers variadic functions in C to be a high-severity security risk.[2]
Infunctional programming languages, variadics can be considered complementary to theapply function, which takes a function and a list/sequence/array as arguments, and calls the function with the arguments supplied in that list, thus passing a variable number of arguments to the function.[citation needed] In the functional languageHaskell, variadic functions can be implemented by returning a value of atype classT; if instances ofT are a final return valuer and a function(T t) => x -> t, this allows for any number of additional argumentsx.[further explanation needed]
A related subject interm rewriting research is calledhedges, orhedge variables.[3] Unlike variadics, which are functions with arguments, hedges are sequences of arguments themselves. They also can have constraints ('take no more than 4 arguments', for example) to the point where they are not variable-length (such as 'take exactly 4 arguments') - thus calling themvariadics can be misleading. However they are referring to the same phenomenon, and sometimes the phrasing is mixed, resulting in names such asvariadic variable (synonymous to hedge). Note the double meaning of the wordvariable and the difference between arguments and variables in functional programming and term rewriting. For example, a term (function) can have three variables, one of them a hedge, thus allowing the term to take three or more arguments (or two or more if the hedge is allowed to be empty).
To portably implement variadic functions in theC language, the standardstdarg.h header file is used. The oldervarargs.h header has beendeprecated in favor ofstdarg.h. In C++, the header filecstdarg is used.[4]
#include<stdarg.h>#include<stdio.h>doubleaverage(intcount,...){va_listap;intj;doublesum=0;va_start(ap,count);/* Before C23: Requires the last fixed parameter (to get the address) */for(j=0;j<count;j++){sum+=va_arg(ap,int);/* Increments ap to the next argument. */}va_end(ap);returnsum/count;}intmain(intargc,charconst*argv[]){printf("%f\n",average(3,1,2,3));return0;}
This will compute the average of an arbitrary number of arguments. Note that the function does not know the number of arguments or their types. The above function expects that the types will beint, and that the number of arguments is passed in the first argument (this is a frequent usage but by no means enforced by the language or compiler). In some other cases, for exampleprintf, the number and types of arguments are figured out from a format string. In both cases, this depends on the programmer to supply the correct information. (Alternatively, asentinel value likeNULL ornullptr may be used to indicate the end of the parameter list.) If fewer arguments are passed in than the function believes, or the types of arguments are incorrect, this could cause it to read into invalid areas of memory and can lead to vulnerabilities like theformat string attack. Depending on the system, even usingNULL as a sentinel may encounter such problems;nullptr or a dedicated null pointer of the correct target type may be used to avoid them.
stdarg.h declares a type,va_list, and defines four macros:va_start,va_arg,va_copy, andva_end. Each invocation ofva_start andva_copy must be matched by a corresponding invocation ofva_end. When working with variable arguments, a function normally declares a variable of typeva_list (ap in the example) that will be manipulated by the macros.
va_start takes two arguments, ava_list object and a reference to the function's last parameter (the one before the ellipsis; the macro uses this to get its bearings). InC23, the second argument will no longer be required and variadic functions will no longer need a named parameter before the ellipsis.[note 1][6] It initialises theva_list object for use byva_arg orva_copy. The compiler will normally issue a warning if the reference is incorrect (e.g. a reference to a different parameter than the last one, or a reference to a wholly different object), but will not prevent compilation from completing normally.va_arg takes two arguments, ava_list object (previously initialised) and a type descriptor. It expands to the next variable argument, and has the specified type. Successive invocations ofva_arg allow processing each of the variable arguments in turn. Unspecified behavior occurs if the type is incorrect or there is no next variable argument.va_end takes one argument, ava_list object. It serves to clean up. If one wanted to, for instance, scan the variable arguments more than once, the programmer would re-initialise yourva_list object by invokingva_end and thenva_start again on it.va_copy takes two arguments, both of themva_list objects. It clones the second (which must have been initialised) into the first. Going back to the "scan the variable arguments more than once" example, this could be achieved by invokingva_start on a firstva_list, then usingva_copy to clone it into a secondva_list. After scanning the variable arguments a first time withva_arg and the firstva_list (disposing of it withva_end), the programmer could scan the variable arguments a second time withva_arg and the secondva_list.va_end needs to also be called on the clonedva_list before the containing function returns.C# describes variadic functions using theparams keyword. A type must be provided for the arguments, althoughobject[] can be used as a catch-all. At the calling site, you can either list the arguments one by one, or hand over a pre-existing array having the required element type. Using the variadic form isSyntactic sugar for the latter.
usingSystem;classProgram{staticintFoo(inta,intb,paramsint[]args){// Return the sum of the integers in args, ignoring a and b.intsum=0;foreach(intiinargs)sum+=i;returnsum;}staticvoidMain(string[]args){Console.WriteLine(Foo(1,2));// 0Console.WriteLine(Foo(1,2,3,10,20));// 33int[]manyValues=newint[]{13,14,15};Console.WriteLine(Foo(1,2,manyValues));// 42}}
The basic variadic facility in C++ is largely identical to that in C. The only difference is in the syntax, where the comma before the ellipsis can be omitted. C++ allows variadic functions withoutnamed parameters but provides no way to access those arguments sinceva_start requires the name of the last fixed argument of the function.
#include<iostream>#include<cstdarg>voidsimple_printf(constchar*fmt...)// C-style "const char* fmt, ..." is also valid{va_listargs;va_start(args,fmt);while(*fmt!='\0'){if(*fmt=='d'){inti=va_arg(args,int);std::cout<<i<<'\n';}elseif(*fmt=='c'){// note automatic conversion to integral typeintc=va_arg(args,int);std::cout<<static_cast<char>(c)<<'\n';}elseif(*fmt=='f'){doubled=va_arg(args,double);std::cout<<d<<'\n';}++fmt;}va_end(args);}intmain(){simple_printf("dcff",3,'a',1.999,42.5);}
Variadic templates (parameter pack) can also be used in C++ with language built-infold expressions.
#include<iostream>template<typename...Ts>voidfoo_print(Ts...args){((std::cout<<args<<' '),...);}intmain(){std::cout<<std::boolalpha;foo_print(1,3.14f);// 1 3.14foo_print("Foo",'b',true,nullptr);// Foo b true nullptr}
TheCERT Coding Standards for C++ strongly prefers the use ofvariadic templates (parameter pack) in C++ over the C-style variadic function due to a lower risk of misuse.[7]
Since the Fortran 90 revision,Fortran functions or subroutines can accept optional arguments:[8] the argument list is still fixed, but the ones that have theoptional attribute can be omitted in the function/subroutine call. The intrinsic functionpresent() can be used to detect the presence of an optional argument. The optional arguments can appear anywhere in the argument list.
programtestimplicit nonereal::x!> all arguments are passed:callfoo(1,2,3.0,4,x)!< outputs 1 \ 2 \ 3.0 \ 4 \ 6.0 (the "\" denotes a newline)!> the last 2 arguments are omitted:callfoo(1,2,3.0)!< outputs 1 \ 2 \ 3.0!> the 2nd and 4th arguments are omitted: the arguments that are positioned after!> an omitted argument must be passed with a keyword:callfoo(1,c=3.0,e=x)!< outputs 1 \ 3.0 \ 6.0!> alternatively, the Fortran 2023 revision has introduced the .NIL. pseudo constant!> to denote an omitted argumentcallfoo(1,.NIL.,3.0,.NIL.,x)!< outputs 1 \ 3.0 \ 6.0contains!> the subroutine foo() has 2 mandatory and 3 optional argumentssubroutinefoo(a,b,c,d,e)integer,intent(in)::ainteger,intent(in),optional::breal,intent(in)::cinteger,intent(in),optional::dreal,intent(out),optional::eprint*,aif(present(b))print*,bprint*,cif(present(d))print*,dif(present(e))thene=2*cprint*,cend if end subroutineend program
Output:
The sum of [1 2] is 3 The sum of [1 2 3] is 6 The sum of [1 2 3 4] is 10
Variadic functions inGo can be called with any number of trailing arguments.[9]fmt.Println is a common variadic function; it uses an empty interface as a catch-all type.
packagemainimport"fmt"// This variadic function takes an arbitrary number of ints as arguments.funcsum(nums...int){fmt.Print("The sum of ",nums)// Also a variadic function.total:=0for_,num:=rangenums{total+=num}fmt.Println(" is",total)// Also a variadic function.}funcmain(){// Variadic functions can be called in the usual way with individual// arguments.sum(1,2)// "The sum of [1 2] is 3"sum(1,2,3)// "The sum of [1 2 3] is 6"// If you already have multiple args in a slice, apply them to a variadic// function using func(slice...) like this.nums:=[]int{1,2,3,4}sum(nums...)// "The sum of [1 2 3 4] is 10"}
Output:
The sum of [1 2] is 3 The sum of [1 2 3] is 6 The sum of [1 2 3 4] is 10
As with C#, theObject type inJava is available as a catch-all.
publicclassProgram{// Variadic methods store any additional arguments they receive in an array.// Consequentially, `printArgs` is actually a method with one parameter: a// variable-length array of `String`s.privatestaticvoidprintArgs(String...strings){for(Stringstring:strings){System.out.println(string);}}publicstaticvoidmain(String[]args){printArgs("hello");// short for printArgs(["hello"])printArgs("hello","world");// short for printArgs(["hello", "world"])}}
JavaScript does not care about types of variadic arguments.
functionsum(...numbers){returnnumbers.reduce((a,b)=>a+b,0);}console.log(sum(1,2,3));// 6console.log(sum(3,2));// 5console.log(sum());// 0
It's also possible to create a variadic function using the arguments object, although it is only usable with functions created with thefunction keyword.
functionsum(){returnArray.prototype.reduce.call(arguments,(a,b)=>a+b,0);}console.log(sum(1,2,3));// 6console.log(sum(3,2));// 5console.log(sum());// 0
Lua functions may pass varargs to other functions the same way as other values using thereturn keyword. tables can be passed into variadic functions by using, in Lua version 5.2 or higher[10]table.unpack, or Lua 5.1 or lower[11]unpack. Varargs can be used as a table by constructing a table with the vararg as a value.
functionsum(...)--... designates varargslocalsum=0for_,vinpairs({...})do--creating a table with a varargs is the same as creating one with standard valuessum=sum+vendreturnsumendvalues={1,2,3,4}sum(5,table.unpack(values))--returns 15. table.unpack should go after any other arguments, otherwise not all values will be passed into the function.functionadd5(...)return...+5--this is incorrect usage of varargs, and will only return the first value providedendentries={}functionprocess_entries()localprocessed={}fori,vinpairs(entries)doprocessed[i]=v--placeholder processing codeendreturntable.unpack(processed)--returns all entries in a way that can be used as a varargendprint(process_entries())--the print function takes all varargs and writes them to stdout separated by newlines
Pascal is standardized byISO standards 7185 (“Standard Pascal”) and 10206 (“Extended Pascal”).Neither standardized form of Pascal supports variadic routines,except for certainbuilt-in routines (read/readLn andwrite/writeLn, and additionally inEPreadStr/writeStr).
Nonetheless,dialects of Pascal implement mechanismsresembling variadic routines.Delphi defines anarrayofconst data type that may be associated with thelastformal parameter.Within the routine definition thearrayofconst is anarrayofTVarRec, anarray ofvariant records.[12]TheVType member of the aforementionedrecord data type allows inspection of the argument’s data type and subsequent appropriate handling.TheFree Pascal Compiler supports Delphi’s variadic routines, too.[13]
This implementation, however, technically requires asingle argument, that is anarray.Pascal imposes the restriction that arrays need to be homogenous.This requirement is circumvented by utilizing a variant record.TheGNU Pascal defines a real variadic formal parameter specification using an ellipsis (...), but as of 2022 no portable mechanism to use such has been defined.[14]
Both GNU Pascal and FreePascal allow externally declared functions to use a variadic formal parameter specification using an ellipsis (...).
PHP does not care about types of variadic arguments unless the argument is typed.
functionsum(...$nums):int{returnarray_sum($nums);}echosum(1,2,3);// 6
And typed variadic arguments:
functionsum(int...$nums):int{returnarray_sum($nums);}echosum(1,'a',3);// TypeError: Argument 2 passed to sum() must be of the type int (since PHP 7.3)
Python does not care about types of variadic arguments.
deffoo(a,b,*args):print(args)# args is a tuple (immutable sequence).foo(1,2)# ()foo(1,2,3)# (3,)foo(1,2,3,"hello")# (3, "hello")
Keyword arguments can be stored in a dictionary, e.g.def bar(*args, **kwargs).
InRaku, the type of parameters that create variadic functions are known asslurpy array parameters and they're classified into three groups:
These parameters are declared with a single asterisk (*) and they flatten arguments by dissolving one or more layers of elements that can be iterated over (i.e,Iterables).
subfoo($a,$b, *@args) {say@args.perl;}foo(1,2)# []foo(1,2,3)# [3]foo(1,2,3,"hello")# [3 "hello"]foo(1,2,3, [4,5], [6]);# [3, 4, 5, 6]
These parameters are declared with two asterisks (**) and they do not flatten any iterable arguments within the list, but keep the arguments more or less as-is:
subbar($a,$b, **@args) {say@args.perl;}bar(1,2);# []bar(1,2,3);# [3]bar(1,2,3,"hello");# [3 "hello"]bar(1,2,3, [4,5], [6]);# [3, [4, 5], [6]]
These parameters are declared with a plus (+) sign and they apply the"single argument rule", which decides how to handle the slurpy argument based upon context. Simply put, if only a single argument is passed and that argument is iterable, that argument is used to fill the slurpy parameter array. In any other case,+@ works like**@ (i.e., unflattened slurpy).
subzaz($a,$b, +@args) {say@args.perl;}zaz(1,2);# []zaz(1,2,3);# [3]zaz(1,2,3,"hello");# [3 "hello"]zaz(1,2, [4,5]);# [4, 5], single argument fills up arrayzaz(1,2,3, [4,5]);# [3, [4, 5]], behaving as **@zaz(1,2,3, [4,5], [6]);# [3, [4, 5], [6]], behaving as **@
Ruby does not care about types of variadic arguments.
deffoo(*args)printargsendfoo(1)# prints `[1]=> nil`foo(1,2)# prints `[1, 2]=> nil`
Rust does not support variadic arguments in functions. Instead, it usesmacros.[15]
macro_rules!calculate{// The pattern for a single `eval`(eval$e:expr)=>{{{letval:usize=$e;// Force types to be integersprintln!("{} = {}",stringify!{$e},val);}}};// Decompose multiple `eval`s recursively(eval$e:expr,$(eval$es:expr),+)=>{{calculate!{eval$e}calculate!{$(eval$es),+}}};}fnmain(){calculate!{// Look ma! Variadic `calculate!`!eval1+2,eval3+4,eval(2*3)+1}}
Rust is able to interact with C's variadic system via ac_variadic feature switch. As with other C interfaces, the system is consideredunsafe to Rust.[16]
objectProgram{// Variadic methods store any additional arguments they receive in an array.// Consequentially, `printArgs` is actually a method with one parameter: a// variable-length array of `String`s.privatedefprintArgs(strings:String*):Unit={strings.foreach(println)}defmain(args:Array[String]):Unit={printArgs("hello");// short for printArgs(["hello"])printArgs("hello","world");// short for printArgs(["hello", "world"])}}
Swift cares about the type of variadic arguments, but the catch-allAny type is available.
funcgreet(timeOfTheDay:String,names:String...){// here, names is [String]print("Looks like we have\(names.count) people")fornameinnames{print("Hello\(name), good\(timeOfTheDay)")}}greet(timeOfTheDay:"morning",names:"Joseph","Clara","William","Maria")// Output:// Looks like we have 4 people// Hello Joseph, good morning// Hello Clara, good morning// Hello William, good morning// Hello Maria, good morning
ATcl procedure or lambda is variadic when its last argument isargs: this will contain a list (possibly empty) of all the remaining arguments. This pattern is common in many other procedure-like methods.[17][18]
procgreet{timeOfTheDayargs}{puts"Looks like we have [llength $args] people"foreachname$args{puts"Hello $name, good $timeOfTheDay"}}greet"morning""Joseph""Clara""William""Maria"# Output:# Looks like we have 4 people# Hello Joseph, good morning# Hello Clara, good morning# Hello William, good morning# Hello Maria, good morning